This invention relates generally to a vehicle automatic transmission and more particularly to a 1-2 shift valve operating in response to a governor pressure counteracting a throttle pressure, and in response to a detent pressure used in a kick-down operation. In a 1-2 shift valve of the prior art, detent pressure, throttle pressure and governor pressure are each applied through different passages. Throttle pressure is applied by way of a ball check valve and changeover between application of the throttle pressure and the detent pressure is effected by the ball of the check valve. However, the performance of the check ball is not sufficiently reliable and adequate sealing of the valve is not achieved. Furthermore, the construction of the conventional 1-2 shift valve is costly. Additionally, the spool of the conventional 1-2 shift valve is difficult to fabricate, has an unavoidably high material cost, and is likely to slide irregularly once assembled because it comprises a single element having a plurality of lands of different areas.
Further, even if the spool of the 1-2 shift valve is fabricated from two discrete component spools, each spool component having lands of the same areas as the one piece spool, only the fabricating procedure, cost and reliability of operation are improved. However, it is unavoidable in the conventional spool of the prior art that the contours of each component spool are not bilateral along the axis of the spool, that is, symmetrical end for end, and this is likely to cause faulty assembly resulting in a malfunction of the valve.
What is needed is a 1-2 shift valve for an automatic transmission having a spool fabricated of independent components and operating without the use of any ball check valve.